An electric linear motion actuator, which is driven by an electric motor, includes a motion converter mechanism configured to convert the rotary motion of the rotor shaft of the electric motor to a linear motion of an axially movably supported driven member.
Ball-screw mechanisms and ball-ramp mechanisms are used as the motion converter mechanisms of known electric linear motion actuators. Such motion converter mechanisms can increase power to some extent, but cannot increase power to a level required in e.g. an electric disk brake system.
Thus, in an electric linear motion actuator in which one of the above-described motion converter mechanisms is used, it is necessary to mount a separate speed reduction mechanism such as a planetary gear mechanism to increase power to a sufficient level. The addition of such a speed reduction mechanism, however, increases complexity and size of the electric linear motion actuator.
In order to avoid this problem, the applicant of the present invention proposed electric linear motion actuators which are capable of increasing power to a sufficient level without mounting a speed reduction mechanism, and thus which are suitable for use in electric disk brake systems, of which a linear stroke is relatively short, in the below-identified Patent documents 1 and 2.
The electric linear motion actuator disclosed in either of Patent documents 1 and 2 includes planetary rollers disposed between a rotary shaft rotated by an electric motor and an axially movably supported outer ring member such that when the rotary shaft is rotated, the planetary rollers revolve around the rotary shaft while rotating about their respective axes due to frictional contact between the planetary rollers and the rotary shaft. The outer ring member has on its radially inner surface a helical rib which is engaged in helical grooves or circumferential grooves formed on the radially outer surfaces of the planetary rollers such that when the planetary rollers revolve around the rotary shaft, the outer ring member is moved in the axial direction.